Molten carbonate fuel cell sulfur scrubber and method using same

ABSTRACT

Sulfur compounds poison catalysts, such as the anode catalysts and reformer catalysts within molten carbonate fuel cell systems. This poisoning is eliminated using a sulfur scrubber 29 located prior to the inlet of the cathode chamber 13. Anode exhaust 19 which contains water, carbon dioxide and possibly sulfur impurities, is combined with a cathode exhaust recycle stream 22 and an oxidant stream 25 and burned in a burner 33 to produce water, carbon dioxide. If sulfur compounds are present in either the anode exhaust, cathode exhaust stream, or oxidant stream, sulfur trioxide and sulfur dioxide are produced. The combined oxidant-combustion stream 27 from the burner 33 is then directed through a sulfur scrubber 29 prior to entering the cathode chamber 13. The sulfur scrubber 29 absorbs sulfur compounds from the combined oxidant-combustion stream 27. Removal of the sulfur compounds at this point prevents concentration of the sulfur in the molten carbonate fuel cell system. Therefore, neither the reformer catalysts nor the anode 17 experience sulfur poisoning.

This is a division of copending application Ser. No. 7/814,520 filed onDec. 30, 1991, now U.S. Pat. No. 5,213,912.

TECHNICAL FIELD

The present invention relates to the removal of sulfur compounds from amolten carbonate fuel cell system, and especially to the removal ofsulfur compounds in the oxidant stream of a molten carbonate fuel cellsystem.

BACKGROUND OF THE INVENTION

Molten carbonate fuel cell systems can be used in the production ofelectricity. These systems typically comprise a reformer for convertinghydrocarbon fuels to hydrogen and byproducts, a burner, and a pluralityof molten carbonate fuel cells. The fuel cells operate such that oxygencontained in an oxidant stream reacts with carbon dioxide and freeelectrons at a cathode to produce carbonate ions. These carbonate ionsmigrate across a molten carbonate electrolyte to an anode where theyreact with hydrogen contained in a fuel stream to produce water, carbondioxide, and free electrons. The free electrons pass through an externalload back to the cathode, thereby producing electricity, while thecarbon dioxide, water, and any remaining hydrogen exit the anode in thefuel stream.

In molten carbonate fuel cell systems, the fuel and oxidant streams areoften contaminated or can become contaminated with sulfur compounds suchas sulfur dioxide, sulfur trioxide, and hydrogen sulfide. These sulfurcompounds can poison various components of the fuel cell systemincluding the anode and the catalyst used in the reformer. The anode isreadily poisoned by contact with sulfur compounds in amounts exceedingabout 1 to 2 parts per million (ppm) by volume while the reformercatalyst is poisoned at very low sulfur concentrations, even below about0.1 ppm by volume. Poisoning the anode reduces its activity andtherefore its ability to convert hydrogen and carbonate ions to water,carbon dioxide, and free electrons while poisoning the reformer catalystreduces its activity and therefore its ability to convert hydrocarbonfuels to hydrogen. As a result of this poisoning, the activity of theanode and the reformer catalyst, and the life of the molten carbonatefuel cell system are all reduced.

Reformer catalyst poisoning has conventionally been eliminated bypurifying the fuel stream prior to its introduction to the reformer.However, the fuel stream is not the only source of sulfur. In a moltencarbonate fuel cell sulfur is also introduced by the oxidant stream.This sulfur can concentrate within the molten carbonate fuel cell andpoison the anode or reformer catalyst within the molten carbonate fuelcell system. In the molten carbonate fuel cell system, the oxidizingconditions at the cathode cause the molten carbonate electrolyte to havea high affinity for sulfur compounds. As a result, the amount of sulfurtrapped within the molten carbonate fuel cell system increases withtime.

Sulfur is typically introduced to the molten carbonate fuel cell systemin the oxidant stream which is directed to the cathode where it isconverted to sulfate ions. The sulfate ions migrate across the moltencarbonate electrolyte to the anode where they are converted withhydrogen to hydrogen sulfide and released into the fuel stream. The fuelstream then exits the anode and is directed to a burner where it isburned. Within the burner, the hydrogen sulfide is converted to sulfurdioxide and sulfur trioxide. The burned stream is then directed alongwith the oxidant stream back to the cathode. Although a portion of thestream exiting the cathode is generally vented, the high affinity andcapture of sulfur at the cathode results in an essentially sulfur-freecathode exhaust stream. Therefore, none of the sulfur is vented. Thesulfur concentration simply continues to build up within the moltencarbonate fuel cell system, thereby compounding the sulfur poisoningproblem.

What is needed in the art is a means for removing sulfur compounds froma molten carbonate fuel cell system to prevent contamination of thereformer catalyst and the anode.

DISCLOSURE OF THE INVENTION

The present invention relates to an improved molten carbonate fuel cellsystem. This system includes a fuel stream inlet, an oxidant streaminlet, and a molten carbonate fuel cell having an anode chamber, acathode chamber, an anode, a cathode and an electrolyte disposedtherebetween and in intimate contact with said anode and cathode. Theimprovement comprises a sulfur scrubber in flow communication withoxidant stream inlet and said cathode chamber.

The present invention further relates to the removal of sulfur from amolten carbonate fuel cell system. This removal method includesintroducing a fuel stream containing hydrogen to the anode where thehydrogen reacts with carbonate ions to form carbon dioxide, water, andfree electrons, and also reacts with sulfate ions to form hydrogensulfide. The reformed fuel stream is then removed from the anode asanode exhaust. An oxidant stream is introduced to a sulfur scrubberwhich absorbs any sulfur compounds. The scrubbed stream is thenintroduced to the cathode where the oxygen, carbon dioxide, and freeelectrons react to form carbonate ions which migrate across theelectrolyte to react with the hydrogen. The scrubbed stream then exitsthe cathode as a cathode exhaust stream.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The figure is one embodiment of the molten carbonate fuel cell system ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the Figure, which is meant to be exemplary not limiting,the molten carbonate fuel cell system of the present invention includesa reformer 5 having a reformer catalyst, a molten carbonate fuel cell50, a burner 33, and a sulfur scrubber 29. The molten carbonate fuelcell 50 can either be a single molten carbonate fuel cell or a pluralityof molten carbonate fuel cells which are typically electricallyconnected in series. Each fuel cell has an anode chamber 15, a cathodechamber 13, an anode 17, a cathode 11, and a molten carbonateelectrolyte 9 disposed therebetween and in intimate contact with theanode 17 and cathode 11.

Operation of this molten carbonate fuel cell system comprisesintroducing a fuel stream 1 and water vapor to the reformer 5. The fuelstream 1 is generally a hydrogen stream or a conventional hydrocarbonfuel such as naphtha, natural gas, propane, coal gas, and others. Sincethis fuel typically contains sulfur impurities, it is preferred topretreat the fuel prior to introducing it to the molten carbonate fuelcell system, thereby reducing poisoning of the reformer catalyst. Itshould be noted that even the pretreated fuel stream typically containssome sulfur impurities. However, this source of sulfur should be minimaland less than about 5% of the total sulfur introduced to the system,typically equal to or less than about 0.01 ppm.

The water vapor entering the reformer 5 is generally provided byrecycling a large portion of the anode exhaust 19 to the reformer 5 viaanode exhaust recycle stream 16. Additional water vapor can beintroduced with the fuel stream if desired.

Within the reformer 5, the combined fuel and anode exhaust recyclestream 3 intimately contacts the reformer catalyst and reacts in anendothermic reaction to form hydrogen, carbon dioxide and carbonmonoxide. Heat for this endothermic reaction is preferably provided bythe waste heat in the anode exhaust recycle stream 16 which is atelevated temperatures, typically between about 1100° F. (593° C.) andabout 1300° F. (705° C.). The reformer catalyst is a conventional steamreformer catalyst such as nickel based catalysts, and noble metal basedcatalysts such as platinum, ruthenium, palladium, rhodium, among others,while the reformer 5 is an adiabatic reformer which uses sensible heatin the stream entering the reformer to provide the endothermic heatrequired for reforming the hydrocarbon fuel to hydrogen, carbon dioxide,and carbon monoxide.

The reformer 5 is typically located in an anode gas recycle loop 40prior to the molten carbonate fuel cell 50 such that the fuel isconverted to hydrogen prior to entering the fuel cell. It is preferredto locate the reformer 5 external to the molten carbonate fuel cell 50,thereby isolating the reformer catalyst from the molten carbonateelectrolyte which can poison it. Additionally, such a location allowsthe use of larger volumes of catalyst than can conveniently fit withinthe anode cavity, thereby reducing the frequency in which this catalystmust be replaced. Finally, locating the reformer 5 external to themolten carbonate fuel cell allows easier replacement of the reformercatalyst. However, the spirit and scope of the present invention are notaffected by the placement of the reformer.

From the reformer 5, the reformed fuel stream 7 is directed to the anodechamber 15 where it contacts the anode 17 which is a conventionalcatalyst for use within a molten carbonate fuel cell. At the anode 17,the hydrogen reacts with carbonate ions to form carbon dioxide, water,and free electrons, and, if sulfate ions are present, the hydrogenreacts with these sulfate ions to form hydrogen sulfide. The hydrogensulfide, water, carbon dioxide, hydrogen, and byproducts then exit theanode chamber 15 as anode exhaust 19. A large portion of this anodeexhaust 19 may be recycled to the reformer 5 while the remainder anodeexhaust stream 20 can be directed to the burner 33.

The remainder anode exhaust stream 20 may be combined with a cathodeexhaust recycle stream 22. This cathode exhaust recycle stream 22supplies oxygen necessary for operation of the burner 33. This combinedstream 24 is then directed to the burner 33 where it is burned to formwater, carbon dioxide, sulfur trioxider sulfur dioxide and otherbyproducts. The burner 33 is typically a catalytic burner containing acombustion catalyst in the form of pellets or honeycomb monolith. Thisburner is typically operated at temperatures between about 1100° F.(593° C.) and about 1600° F. (870° C.).

An oxidant stream 25, which is any oxygen containing stream, can eitherbe combined with the remainder anode exhaust stream 20 and the cathodeexhaust recycle stream 22 prior to the burner or it can be introduced tothe combustion stream 23 exiting the burner 33. Generally, the oxidantstream 25 is an air stream which may also contain sulfur impurities,typically in the form of sulfur dioxide in concentrations between about10 to about 50 parts per billion (ppb) by volume. Although this sulfurdioxide concentration is small, the total amount of sulfur introducedinto the molten carbonate fuel cell system in the air is often asignificant amount since a large volumetric flow of air is used as bothoxidant and for cooling the fuel cell. Sulfur introduced in the air canrepresent greater than about 95% of the potential sulfur introduced intothe system by both the fuel and oxidant streams. Therefore, scrubbingthe oxidant stream prior to introducing it to the cathode chamber 13 isextremely important in diminishing poisoning problems in the moltencarbonate fuel cell system.

In order to remove the sulfur from the oxidant str it is directed to thesulfur scrubber 29 where it contacts a sorbent material capable ofabsorbing sulfur compounds. Since the combustion stream 23 may alsocontain some sulfur impurities which entered the molten carbonate fuelcell system in the fuel stream 1 or which were not removed in thescrubber and passed to the fuel stream through the molten carbonateelectrolyte, it is preferred to pass the combined oxidant-combustionstream 27 through the sulfur scrubber 29. By introducing a combinedoxidant-combustion stream 27 to the sulfur scrubber 29, the sulfurscrubber 29 is able to scrub all sources of sulfur within or enteringthe molten carbonate fuel cell system, thereby minimizing and limitingthe accumulation of sulfur in the molten carbonate fuel cell system.Essentially all of the sulfur entering the molten carbonate fuel cellsystem is removed in the sulfur scrubber 29.

Sorbent materials used in the sulfur scrubber 25 which are particularlyuseful with the present invention include alkali carbonates such ascarbonates of lithium, potassium, sodium, and others which formsulfates. These carbonates are typically in the form of moltencarbonates which would be supported on a ceramic material such aslithium aluminate. Generally, this support is in the form of pellets ormonolithic honeycomb.

The sorbent material absorbs sulfur compounds, particularly sulfuroxides. These sulfur oxides are eventually absorbed as alkalinesulfates. Since sulfur trioxide readily reacts with the sorbent materialto form sulfates, conversion of the sulfur compounds, especially sulfurdioxide, to sulfur trioxide will improve the efficiency of the sulfurscrubber 29. Therefore, the use of the sorbent material in combinationwith a catalyst capable of converting sulfur dioxide to sulfur trioxideis preferred. This catalyst is typically interspersed with the sorbentmaterial and serves to enhance and complete the reaction of sulfurdioxide with oxygen to sulfur trioxide. The sulfur trioxide then reactswith the sorbent material to form a sulfate which is the most stableform of the absorbed sulfur and, therefore, the most desirable form.

This scrubber catalyst can be any conventional catalyst capable ofconverting sulfur dioxide to sulfur trioxide. Some possible catalystsinclude: nickel based catalysts such as nickel oxide, noble metal basedcatalysts such as platinum and silver, and other metallic catalysts suchas copper based catalysts, vanadium based catalysts, mixtures thereof,and others. Vanadium pentoxide is used industrially for the oxidation ofsulfur dioxide to sulfur trioxide.

The sulfur scrubber 29 is preferably operated at temperatures rangingfrom about 1000° F. (538° C.) to about 1300° F. (705° C.), with atemperature between about 1100° F. (593° C.) and about 1200° F. (649°C.) especially preferred. If the sorbent material is molten carbonate,temperatures below about 1000° F. (538° C.) will cause the moltencarbonate to solidify thereby reducing the sulfur scrubber 29effectiveness while temperatures above about 1300° F. (705° C.) exceedthe temperature limit of the molten carbonate fuel cell 50.

Eventually, the sorbent material within the sulfur scrubber 29 becomessaturated with sulfur compounds. For example, essentially all of themolten carbonate reacts with sulfur trioxide,, thereby converting all ofthe molten carbonate to sulfates. As a result, the sorbent material isreplaced. It is foreseen that the sulfur material within the sulfurscrubber 29 could be regenerated and that a plurality of sulfurscrubbers could be employed with the present invention. If a pluralityof sulfur scrubbers are utilized, a portion of these scrubbers willabsorb sulfur compounds while the remainder of these scrubbers will beregenerated. Although such a system is possible, it is not presentlyconsidered cost effective since the cost of sorbent materials isrelatively low while the cost of a system for regenerating the sorbentmaterial is comparatively high. Such a system may, however, be usefulwhere cost is a secondary issue and volume and/or weight considerationsare paramount.

Once the sulfur compounds have been removed from the combinedoxidant-combustion stream 27 in the sulfur scrubber 29, the scrubbedstream 31 is introduced to the cathode chamber 13 of the moltencarbonate fuel cell 50. At the cathode 11, in the cathode chamber 13,the scrubbed stream 31 intimately contacts a conventional catalyst whereoxygen and carbon dioxide in the scrubbed stream 31 react with freeelectrons which have passed from the anode 17 to the cathode 11 throughan external load, to produce carbonate ions. These carbonate ionsmigrate across the molten carbonate electrolyte 9 to the anode 17 wherethey react with hydrogen in the formation of water, carbon dioxide, andfree electrons. Meanwhile, the scrubbed stream 31 exits the cathodechamber 13 as cathode exhaust stream 21 some of which is vented and theremainder is recycled and combined with the anode exhaust 20.

Advantages of the present invention are readily apparent. The presentinvention is an efficient, effective method of removing sulfur from amolten carbonate fuel cell system and therefore results in an effectivemolten carbonate fuel cell system. Unlike the prior art, sulfurcompounds are not concentrated within the molten carbonate fuel cellsystem. As a result, sulfur poisoning of the anode is essentiallyeliminated, sulfur poisoning of the reformer catalyst is significantlyreduced if not eliminated, and the life of the overall molten carbonatefuel cell system is increased.

The present system further reduces maintenance requirements. Since thelife of the reformer catalyst and anode is increased, the period betweeninstallation and replacement is similarly increased. The presentinvention significantly improves and simplifies the molten carbonatefuel cell system of the prior art.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the invention.

We claim:
 1. A method for removing sulfur from a molten carbonate fuelcell system, said system having a burner, a molten carbonate fuel cell,said molten carbonate fuel cell having an anode, a cathode, a moltencarbonate electrolyte disposed therebetween, an anode chamber and acathode chamber, which comprises the steps of:a. introducing a fuelstream containing hydrogen to the anode chamber where said fuel systemcontacts said anode, b. reacting said hydrogen at said anode withcarbonate ions to form carbon dioxide, water and free electrons, wheresulfur compounds in said fuel stream and on the anode react withhydrogen to form hydrogen sulfide; c. removing the fuel stream,including the carbon dioxide, water, and any hydrogen sulfide from theanode chamber as anode exhaust, d. introducing an oxidant stream to asulfur scrubber wherein said sulfur scrubber removes sulfur compoundsfrom said oxidant stream to form a scrubbed stream; e. introducing saidscrubbed stream to the cathode chamber where said scrubbed streamcontacts said cathode; f. reacting carbon dioxide and O₂ in saidscrubbed stream with said free electrons which passes from the anode tothe cathode to produce carbonate ions which migrate across the moltencarbonate electrolyte to the anode; and g. removing said scrubbed streamfrom said cathode chamber as a cathode exhaust stream.
 2. A method as inclaim 1 wherein said sulfur scrubber uses a sorbent material whichabsorbs sulfur compounds to remove the sulfur compounds from the oxidantstream.
 3. A method as in claim 2 wherein said sorbent material isselected from the group consisting of alkali metal carbonates.
 4. Amethod as in claim 2 wherein said sulfur scrubber further uses acatalyst to convert said sulfur compounds to sulfur trioxide.
 5. Amethod as in claim 4 wherein said catalyst is selected from the groupconsisting of nickel based catalysts, noble metal based catalysts,copper based catalysts, vanadium based catalysts, and mixtures thereof.6. A method as in claim 1 further comprising the steps of:a. combiningsaid anode exhaust with a cathode exhaust recycle stream; b. introducingsaid combined stream to the burner; and c. burning said combined streamto produce a combustion stream.
 7. A method as in claim 6 furthercomprising the steps of introducing said combustion stream to thecathode chamber.
 8. A method as in claim 6 further comprising the stepsof combining said combustion stream with said oxidant stream prior tosaid sulfur scrubber.
 9. A method as in claim 6 further comprising thesteps of combining said oxidant stream with said combined stream priorto said burner.
 10. A method as in claim 1 further comprising the stepsof introducing the fuel stream to a reformer prior to introducing saidfuel stream to said anode chamber, wherein said fuel stream ishydrocarbon fuel which is converted to hydrogen, carbon dioxide, andcarbon monoxide in said reformer.
 11. A sulfur scrubber which comprisesa molten carbonate sorbent material supported on a ceramic support,wherein said scrubber is operated at temperatures between about 1000° F.and about 1300° F.
 12. A sulfur scrubber as in claim 11 wherein saidmolten carbonate sorbent material is selected from the group consistingof alkali metal carbonates.
 13. A sulfur scrubber as in claim 11 furthercomprising a catalyst interspersed with said molten carbonate sorbentmaterial, wherein said catalyst converts sulfur oxides to sulfurtrioxide.
 14. A sulfur scrubber as i claim 13 wherein said catalyst isselected from the group consisting of nickel based catalysts, noblemetal based catalysts, copper based catalysts, vanadium based catalysts,and mixtures thereof.
 15. A sulfur scrubber as in claim 11 wherein saidceramic support is lithium aluminate.
 16. A sulfur scrubber comprising acatalysts interspersed with a molten carbonate sorbent material, whereinsaid catalyst converts sulfur oxides to sulfur trioxide.
 17. A sulfurscrubber as in claim 16 wherein said molten carbonate material is analkali metal carbonate supported on a lithium aluminate support.
 18. Asulfur scrubber as in claim 16 wherein said catalyst is a nickel basedcatalyst, a noble metal based catalyst, a copper based catalyst, avanadium based catalysts, or a mixture thereof.